This invention relates to the modification of starch content of plants, and in particular, to the increase of starch content in plants.
Starch is a complex polymer of glucosyl residues. It is the major form in which carbohydrate is stored in the tissues of most species of higher plants. It is accumulated in the leaves of plants during the day as a result of photosynthesis and is used to supply the needs of the plant for energy and biosynthesis during the night. Starch is also accumulated in non-photosynthetic tissues, especially those involved in reproduction such as seeds, fruits and tubers. Therefore, starch is of great importance to the productivity of the plant and its survival.
Starch is also highly significant to man. Firstly, it forms a major component of animal diets, supplying man and his domestic animals with a large portion of their carbohydrate intake. Secondly, the type of starch in a plant affects the quality of the processed plant product. Thirdly, starch is used industrially in the production of paper, textiles, plastics and adhesives, as well as providing the raw material for some bio-reactors. Starch from different species have preferred uses. On a world scale, starch producing crops are agriculturally and economically by far the most important, and these crops include wheat, maize, rice and potatoes. The quantity of starch present in the harvested organ of a plant will affect the gross yield and the processing efficiency of the crop. In addition, the type of starch will affect the quality of a processed product and the profitability of the process.
Starch is synthesised in amyloplasts in plants from glucose-1-phosphate (Glc-1-P) as shown below. 
[EC.2.7.7.27] (ADPG PPase) catalyses the first committed step of the pathway of starch biosynthesis in plants. A similar enzyme catalysing the same reaction is found in bacteria and cyanobacteria.
The quaternary structure of the enzyme is similar in all organisms investigated in that the functional enzyme is composed of a tetramer of subunit proteins. In bacteria the protein subunits are identical and the product of a single gene, e.g. in E. coli the GlgC gene. In plants, however, the enzyme is composed of two each of two different protein subunits. While these different protein subunits display sequence similarities, they are the product of two distinct genes.
There are many mutants of plants that have a lower starch content in particular tissues compared to that of wild-type plants. These mutant plants are deficient in the expression of one of the genes coding for the subunits of ADPG PPase. Two particular mutations seen in maize endosperm are the mutants shrunken-2 and brittle-2. It is argued that the wild-type genes for these code for the two subunit proteins of the enzyme. Both mutations cause decreased enzyme activity of ADPG PPase in the endosperm. It is argued from this information that both subunits of the enzyme are required for full activity and that lack of a particular type of subunit cannot be compensated for by the other subunit.
This invention is based on the fact that one only of the genes for one of the subunit proteins of an enzyme catalysing starch production is required to increase enzyme activity.
It is an object of the present invention to provide a method for increasing the activity of an enzyme catalysing starch synthesis.
It is a further object of the present invention to provide a plant having an increased starch content when compared with a control plant not treated in accordance with the inventive method.
It is also an object of the invention to increase the rate of starch synthesis under conditions which do not lead to a compensating increase in the rate of starch breakdown.
The present invention provides a method of increasing the enzyme activity in a plant comprising introducing into a plant one of the genes of one of the subunit proteins of an enzyme catalysing starch synthesis, thereby causing expression of the subunit gene in the plant to produce the subunit protein, and an increase in the enzyme activity in the plant cells.
The present invention further provides a plant into which has been introduced one of the genes of one of the subunit proteins of an enzyme catalysing starch synthesis, which plant expresses the gene to produce a subunit protein, and increases the enzyme activity in the plant cells.
The method may also include introducing one of the genes of one of the subunit proteins of a plurality of other enzymes catalysing starch synthesis.
The present invention also provides a plasmid incorporating one of the genes of one of the subunit proteins of an enzyme catalysing starch synthesis in plants.
A plasmid or plant according to the invention may also contain one of the genes of one of the subunit proteins of one or more other enzymes catalysing starch synthesis in the plants.
The present invention also provides a plant cell harbouring a plasmid described above.
Preferably the gene is the brittle-2 gene or a homologue thereof. Advantageously the ;gene is the wheat brittle-2 gene.
Preferably the plant is grown commercially and is any one of maize, wheat, rice, potato, cassava, peanut, beans, carrots, tomato or tobacco crop for example.
Preferably ADPG PPase activity is increased by the method of the invention.
An increase in starch content, especially in potatoes, may be measured as an increase in specific gravity (S.G.) of the plant or tuber, for example.
Preferably the plasmid incorporates a homologue of the brittle-2 gene of an enzyme catalysing starch synthesis. Alternatively, the plasmid may incorporate a homologue of the shrunken-2 gene of an enzyme catalysing starch synthesis.
In order that the present invention may be easily understood and readily carried into effect reference will now be made to the following Example and the drawings,